The proposed research will contribute to the understanding of human lightness perception. Recent work on lightness perception has suggested that many phenomena may be understood in terms of anchoring and perceptual grouping. The essential ideas are: (1) The luminance distribution of a scene serves to anchor and scale local luminance measurements into a particular lightness mapping. (2) Perceptual grouping of luminance patches establishes boundaries that affect the spatial weighting scheme of the lightness mapping. The investigators will perform experiments on lightness perception to test the existing models, to help formalize the notions of anchoring and grouping, and to determine how models may be modified or improved in order to account for a wider range of phenomena. The anchoring problem is conceptually recast in a framework of "atmospheres." This approach seeks to estimate lighting and viewing condition parameters from luminance statistics and configurations in order to establish mappings between image luminances and perceived reflectances. The proposed experiments will employ computer-generated and physical stimuli to measure lightness mappings under a wide variety of conditions. Stimuli will be tested both in traditional displays and in specially-designed panoramic displays that permit control of all image luminance values. Anchoring experiments will investigate statistical aspects of luminance distributions that are critical in determining the lightness mapping. Grouping experiments will test the importance of particular junction and contour configurations in establishing boundaries for lightness perception. Additional experiments will investigate the effects of naturalistic cues in everyday scenes in order to identify further mechanisms that contribute to lightness perception. The results of these investigations should be of interest to researchers in lightness, color, and perceptual organization.